Neuroscientists found 'no evidence' that smoking pot damages an important brain region

A new study published today in the journal JAMA Psychiatry casts doubts on the idea that marijuana causes certain regions of the brain to shrink.

In order to study the brain on pot, neuroscientists studied siblings or twins, one of whom was a pot smoker (some regularly, some less so, though none at the “problematic” level) and the other who had never touched the stuff.

If the pot-using sibling had brain changes while the other didn’t, that would imply that marijuana use had caused those differences to appear.

The researchers were specifically interested in the amygdala, because of its association with emotion and motivation and the fact that previous research had shown this area was smaller in pot users.

But, surprisingly, they didn’t find any differences between pot smokers and their abstemious siblings.

“We found no evidence for the causal influence of cannabis exposure on amygdala volume,” the authors write in the paper.

In other words, that important region of the brain would have been the same size, whether those study participants had smoked pot or not. This blows a hole in the theory that casual marijuana use damages the brain — though there are still questions to answer about marijuana’s specific effects.

What they looked at exactly

Dr. David Pagliaccio of the National Institute of Mental Health and co-authors used publicly available data from the Human Connectome Project for the study.

They found 483 participants that had a sibling or twin who had also had their brain mapped as part of the project, all between 22 and 35 years old. Of that group, 262 had used cannabis.

Study participants who’d used pot had on average, a slightly smaller amygdala and a slightly smaller right ventral striatum (another brain region, associated with the reward system).

They didn’t find a statistically significant difference in the size of the hippocampus, another area that’s frequently discussed when talking about the effects of marijuana.

And it’s important to note that we’re not talking impaired-brain levels of smaller here — everything was within the normal human spectrum, though the consistently smaller regions are noteworthy.

It didn’t matter whether users had smoked more than 100 times (30% of the group) or had starting using pot by the time they were 17 (49%) — none of that was associated with more changes than those experienced by occasional or casual users.

But what’s really fascinating is that the pot smokers’ sober siblings showed the same smaller brain regions, even though they’d never smoked pot (people who claimed they hadn’t smoked pot but had tested positive for THC were excluded from the research).

So what does that mean?

In the case of the amygdala, the researchers write that the smaller brain region may be explained by genetic factors — their analysis didn’t show a significant contribution from environment. But, as Francesca Filbey, a researcher and associate professor in the School of Behavioural and Brain Sciences at the University of Texas at Dallas, who was not involved in the research, tells us, it’s very hard to completely control for childhood environment, which likely has a strong influence on brain development.

What they can’t explain is the smaller ventral striatum region in marijuana users and their siblings. They haven’t ruled out any combination of factors for brain differences there, but some other research has shown that region to be larger in occasional marijuana users, which they say complicates the discussion. More research will be needed to figure that out.

But this does help start to answer the question of whether pot causes brain changes or whether people who are likely to smoke pot have different brains in the first place. In the case of the amygdala, this research implies that it’s the latter.

Reasons to be cautious

Even with this step forward, we are still far from explaining all the effects that marijuana has on the brain.

An additional study published in the same issue of JAMA Psychiatry shows increasing risk for schizophrenia among young men (though not women) who are already at high risk for that mental illness and who smoke pot. But that research has the same chicken-and-egg problem as previous studies: the work can’t say whether pot causes those risks to increase, or whether people are more likely to smoke pot as their risk increases.

As Dr. David Goldman of the National Institute of Alcohol Abuse and Alcoholism writes in an accompanying editorial also published in JAMA Psychiatry, this shouldn’t be taken as an endorsement that cannabis is safe.

Pagliaccio and co-authors note that even though this is the largest study to examine the causal effects of smoking marijuana, there are still groups that weren’t included in the study that would be important to analyse.

They didn’t look at any cannabis users who had been hospitalized for two or more days for substance abuse, nor did they include anyone who had been in substance abuse treatment for a year or longer — it’s possible that they would have seen different effects among marijuana users in that group.

The fact that these users were not necessarily chronic or heavy users may have had an impact on the results, according Filbey. “It is possible that any effects of marijuana may not have been detected because the sample may not be heavy users … 100 times in a lifetime is not much,” she tells Tech Insider.

Still, those caveats noted, Filbey does say that the “study shows that marijuana did not influence any changes” in the amygdala, at least for casual pot smokers.

Additionally, the researchers only looked at a few regions of the brain: The hippocampus, right ventral striatum, and amygdala. There’s a lot more going on in the brain outside of those areas, and it’s likely that everything we do, from the food we eat to the music we listen to, has some effect somewhere. We know, for example, that alcohol use changes the brain.

A better understanding of whether or not marijuana has a significant impact on the brain is incredibly important as states around the country continue to discuss legalization of the drug.